Because of the requirements for an improvement of display quality of liquid crystal display devices and weight saving thereof, there has been increasing a demand of a polymer film whose inner molecular orientation structure is controlled for use as an optical compensation film such as a polarizer, a wave plate or the like. In order to meet the above demand, films making use of optical anisotropy of polymerizable liquid crystal compounds have been developed.
The polymerizable liquid crystal compounds used herein are generally those liquid crystal compounds having both a polymerizable group and liquid crystal structural units (i.e. structural units having a spacer moiety and a mesogenic moiety), in which an acrylic group has been widely used as the polymerizable group.
In general, such polymerizable liquid crystal compounds are converted to polymers (films) according to a method of polymerization by irradiation of a radiation such as UV light or the like.
For instance, there are known methods including a method wherein a specific type of polymerizable liquid crystal compound having an acrylic group is held between supports and a radiation is irradiated while keeping the compound in liquid crystal state (see Patent Document 1) and a method wherein a photopolymerizable initiator is added to a mixture of two types of polymerizable liquid crystal compounds having an acrylic group or a composition obtained by further mixing a chiral liquid crystal compound with the mixture, followed by irradiation of UV light to obtain a polymer (see Patent Document 2).
The polymers (films) obtained by the above methods have been used, as a film for polarizer or wave plate, not only in display devices such as a monitor and television set, but also in display devices employed in a high temperature environment such as of automobiles. To this end, to keep transparency under high temperature environment is very important for use as a display device material.
Recently, in the field of displays, investigations for process simplification making use of these materials as an in-cell phase difference film have been actively conducted. As to the materials used in the in-cell technique, higher thermal stability and chemical resistance have been required.
On the other hand, when compared with optical anisotropic films obtained from other processes (e.g. stretching of polyvinyl alcohol films), the films obtained from polymerizable liquid crystal compounds have a problem in that the optical anisotropy thereof is lower in wavelength dependence.